The present invention relates to a ceramic composite having the features given in the preamble of claim 1 as well as a method for producing a ceramic heating element having the features given in the preamble of patent claim 7.
Ceramic composites of the type according to the definition of the species are used, for example, for ceramic heating elements in electrochemical sensors. For this purpose, meander-patterned electrical resistor runs are provided and used to form a ceramic heating element. Electrochemical sensors of this type make it possible to measure an oxygen concentration in an exhaust gas of internal combustion engines to select the setting of a fuel/air mixture for operating the internal combustion engine. In the active range, the sensors must be heated to temperatures above roughly 300xc2x0 C. to achieve the necessary ion conductivity of a solid electrolyte. The ceramic heating element integrated into the electrochemical sensor is used for this purpose.
The production of sensors of this type in the form of planar lambda probes, using a layer technique, is known. To do this, individual layers are arranged on top of each other and possibly patterned. This layer construction is obtained, for example, by sheet casting, stamping, screen-printing, lamination, cutting, sintering or similar methods. The heating elementxe2x80x94in particular, the meandering electrical resistor runs that form the heating elementxe2x80x94is constructed in the same manner. To prevent reductions in adjacent layers, or to suppress leakage currents, the heating element must be shielded by providing an insulating layer.
To increase sensor measurement accuracy, known methods involve the control and possibly adjustment of the sensor operating temperature. In known methods, the heating element is assigned a measuring element via which the heating element can be engaged and disengaged, respectively, as a function of a measured operating temperature. The heating element resistance (according to the present invention, this resistance is the internal electrical resistance of one electrical conductor of the heating element) must lie within narrow tolerances to prevent heating element overloading and underloading, respectively. Otherwise, the detected measured value would be corrupted.
The heating element resistance has proven to vary by an especially large amount in the region of its meandering pattern, due to the manufacturing process. The resistance depends on the temperature, resistance coefficients of the material used, and the length of the heating element conductors. For production reasons, the composition, and thus the resistance coefficient, of individual conductor areas in the heating element can vary, and the heating element conductors may also vary in length. It is not possible to trim the resistance in previously known methods. Consequently, sensors whose resistance proves to be unusable for the heating element during the first measurement must be discarded.
The embodiment described above by way of an example, in which the ceramic heating element forms part of the sensing element of a sensor to measure an oxygen concentration, serves only to explain the disadvantages of the related art. These disadvantages also arise in other applications that use a resistor meander integrated into a ceramic composite. Such examples can include temperature sensors or passive sensors that respond to resistance changes in media. These applications also require a defined resistance of an electrical resistor run.
The ceramic composite according to the present invention having the features given in claim 1 provides the advantage that it includes an integrated electrical resistor run that has a defined, reproducible electrical resistance. Because the layer covering the resistor run has at least one opening through which the resistor run can be trimmed, it is possible to set the resistance of the resistor run at a later time, i.e., after patterning the ceramic composite. The resistor run, which is preferably designed as a resistor meander, has junctions and/or sealed zones (also referred to hereinafter as seal zones, zones, and filled zones) at least in the area of adjacent conductor segments, with the resistance of the resistor run being adjustable by cutting the junctions and/or zones. In an embodiment of this type, a pattern that supports subsequent trimming of the resistor run resistance can be integrated with an easily reproducible layout into the ceramic composite. The junctions and/or sealed zones between adjacent conductor segments are made of the same resistive material as the resistor runs and are patterned together with the latter, in particular, by screen-printing or a similar technique.
The method according to the present invention for producing a ceramic heating element having the features given in claim 7 has the advantage that it can be used to provide mass-produced ceramic heating elements that have a uniform heating meander resistance. Because the electrical resistance of the heating meander is set after sintering the composite, with an effective length of a conductor forming the heating meander preferably being adjusted subsequently, production-related tolerances in the heating meander resistance can be easily equalized. When using such ceramic heating elements according to specification, it is therefore possible, in particular, to combine the heating elements with a measurement and control circuit, thus providing a precise, defined, and reproducible heating meander resistance for the measurement and control circuit. As a result, it is possible to achieve uniform measurement and control results when the ceramic heating elements are mass-produced, since production-related resistance fluctuations that would lead to deviations in the measurement and control results are eliminated.
A preferred application is to use the ceramic composite having the features given in claim 1 as a heating element in a sensing element of an electrochemical sensor, in particular for measuring an oxygen concentration in the exhaust gas of internal combustion engines.
Up to the layer containing the heating element, the sensor is constructed in the known manner by sheet casting, stamping, screen-printing, lamination, cutting, sintering or similar methods. The heating element conductor has junctions and/or filled zones between the individual meander windings in the area with the meandering pattern. The subsequent layers have openings in these exact areas.
After the layers needed to operate the sensor have been applied, the resistance of the heating meander can be trimmed, preferably using a laser, by correcting the length of the heating meander conductors correspondingly. This can be easily accomplished by using a laser to cut or trim the junctions and/or sealed zones between adjacent conductor segments in the heating meander. The openings are then sealed air-tight, for example by glazing. The openings through which laser cutting or laser trimming is carried out are then sealed with a filler, in particular by glazing them with a ceramic glass.
In a preferred embodiment of the present invention, the layers covering the heating element can be designed so that the laser can pass through them to cut or trim the junctions and/or filled zones. As a result of the heat applied to the layer covering the heating meander during laser treatment, heating meanders can be easily glazed at the same time, with these layers containing glazing agents for this purpose.
Further preferred embodiments of the present invention are derived from the remaining features given in the subclaims.